The field of the invention is collagen proteins. The structure of extracellular matrices is to a large extent determined by the interaction between collagenous proteins and proteoglycans. Collagenous proteins form the major class of insoluble fibrous protein in the extracellular matrix. Proteoglycans are complex macromolecules found in the extracellular matrix, connective tissue, and on the surface of many cells. Proteoglycans are composed of a core protein to which is attached one or more polysaccharides known as glycosaminoglycans. Within the extracellular matrix collagen molecules participate in the formation of a polymer framework of high tensile strength, while the proteoglycans, because they are hydrophilic and highly anionic, impart resilience.
Collagen polypeptides contain one or more blocks of Gly-x-y repeats, in which y frequently represents prolyl or hydroxyprolyl residues. The presence of such sequence repeats allows groups of three collagen polypeptides to fold into triple-helical domains which are rigid and inextensible.
Within the superfamily of collagens, the fibrillar collagens represents a distinct family (which includes type-I, type-II, type-III, type-V, and type-XI collagen). The triple-helical domains of the proteins polymerize in a staggered fashion to form fibrils.
Members of other collagen families do not by themselves form cross-striated fibrils, but may be associated with fibrils (FACIT or fibril-associated collagens) or form their own distinct polymers (networks in the case of basement membrane collagens or anchoring fibrils in the case of collagen VII). The lengths as well as the number of triple-helical domains within molecules of non-fibrillar collagens are frequently quite different from these domains in fibrillar collagens.
The non-triple-helical domains that separate triple-helical domains in some non-fibrillar collagens represent regions of flexibility. For example, in types IX, XII and XIV collagen non-triple-helical regions form hinges which allow the triple-helical domains on either side to be oriented in a variety of directions. One function on non-triple-helical domains in such collagen types may be to provide for flexibility between rigid triple-helical regions.